In recent years various devices for protecting vehicle occupants from the impact of a vehicle crash have been proposed and some of them have already been put to actual uses. For instance, various forms of passive seat belt and air bag systems have been proposed for protecting the vehicle occupants from the secondary impact or, in other words, the impact arising from the collision of the vehicle occupant with the interior of the passenger compartment due to the inertia force acting on the vehicle occupant in case of a vehicle crash. These systems have been proven to be effective in reducing the possibility and the degree of injury which would be inflicted upon the vehicle occupant.
As an effort to further promote the safety of vehicle occupants from the secondary impact, it has been proposed to use a knee bolster to support the knees of the vehicle occupant. This is effective because, in case of a vehicle crash, although the upper part of the vehicle occupant is restrained by a safety belt or an air bag from being thrown forward, but the lower part of the body tends to be thrown forward with his knees first and the upper part of his body thus tends to be pulled downward.
However, conventional knee bolsters are not necessarily sufficiently effective. For instance, a simple knee bolster consisting of a steel plate pressformed into a box shape tends to deform easily initially and its resistance to the knee tends to increase sharply as the deformation progresses. When a semi-cylindrical knee bolster is used with its cylindrical surface opposing a knee of a vehicle occupant, an impact from the knee causes a central part of the knee bolster to depress further than those parts located on either lateral side thereof, and this in turn causes a sharp increase in the impact force applied to the knee because further deformation of the knee bolster requires a larger amount of buckling deformation. Further, the deformation of the knee bolster is not necessarily stable and, therefore, the resistance which the knee experience may vary depending on various conditions regarding the way in which the knee hits the knee bolster.
U.S. Pat. No. 4,032,175 proposes a knee bolster comprising a tube member having a rectangular cross section and another tube member having a circular cross section fitted into the interior of the rectangular tube member and welded thereto at appropriate locations. According to this proposal, the deformation of the knee bolster may occur in a stable fashion, but the deformation is still localized and an optimum absorption of impact energy cannot be achieved.